Current instant coffee products are primarily manufactured as spray dried powders. These spray dried powders are typically made by spraying concentrated coffee extract as an atomized fine mist into a spray drying tower many stories high. The air flow within the tower is hot enough to evaporate the residual moisture in the extract so that the falling coffee solids become a dry powder, usually light brown in color, by the time they reach the bottom of the tower. Before spray drying, this concentrated coffee extract is often combined with aroma and flavor volatiles (typically referred to as "stripper condensate") that is removed ("stripped") from the coffee extract before it is concentrated.
Spray drying is a relatively economical way to make instant coffee. However, both the appearance and solubility of these spray dried powders fall short of what is desired by most coffee consumers. These spray dried powders are relatively fine in particle size (e.g., from about 0.1 to about 0.3 mm in diameter) and thus do not look like roast and ground coffee granules. These spray dried powders also tend to dissolve poorly when added directly to hot water. The added powder floats on top of the water, foams, and can form an unsightly ring on the inside of the cup.
Spray drying has the additional adverse effect of driving off important coffee aroma and flavor volatiles. Typically, only about 50 to about 60% of the stripped aroma and flavor volatiles that are combined with concentrated coffee extract are retained after spray drying. This loss of aroma and flavor volatiles detracts from the true coffee flavor of the product. In commercial practice, this volatile loss is compensated for by adding aromatized (coffee) oil to the spray dried coffee powder. Unfortunately, this aromatized oil can cause the spray dried particles to clump together into a less flowable mass that is more difficult to spoon out as a measured quantity of instant coffee.
To improve the appearance and hot water solubility of spray dried coffee powders, these powders are usually agglomerated. Agglomeration is typically carried out by spraying steam onto the spray dried powder. This steam partially melts the spray dried powder so that the particles coalesce and cluster together into larger structures (e.g., about 3 mm in size). These larger agglomerated structures improve the appearance, flowability and solubility of the instant coffee product. Even so, agglomerated spray dried coffee still suffers from the aroma and flavor volatile loss caused by the initial spray drying process. Also, agglomerated spray dried coffee particles are relatively fragile and tend to break up into finer particles in the jar during in-plant handling and during shipment to the trade. The tendency of agglomerates to break-up into finer particles leads to problems in consistently measuring a spoonful of product from the jar. This can make it difficult for the consumer to prepare a cup of coffee having the desired brew strength.
Freeze drying is an alternative commercial process for drying soluble coffee products. Freeze-dried instant coffee is prepared by freezing a coffee extract prepared in a manner the same or similar to that of the extract used in spray drying. This frozen extract, with or without granulation, is placed in a chamber held under a vacuum (e.g., less than 500 micron Hg absolute pressure) and maintained at relatively low temperatures (e.g., less than -15.degree. F. (-26.degree. C.)). The frozen extract is then gently heated to cause sublimation of the ice crystals from the extract, thus removing the water. Because water is removed gently and carefully without significant heating of the frozen extract, freeze dried instant coffees can achieve a high degree of aroma and flavor volatile retention.
Freeze dried instant coffee products can be granulated to provide a chunky physical appearance that is more like that of roast and ground coffee. The sublimation of ice crystals that occurs during freeze drying creates an extremely porous coffee structure. As a result, freeze dried coffee products typically have a lower density than spray dried and especially agglomerated instant coffee products. Freeze dried products typically have poorer solubility than agglomerated spray dried products, especially when added to a cup of hot water. The freeze dried chunks tend to float on the surface and can result in a messy cup preparation. Freeze dried coffee also tends to be lighter in color than spray dried/agglomerated instant coffee products and thus different in color from granular roast and ground coffee. Freeze drying processes are also more capital intensive and costly to run than spray drying/agglomeration processes. This means freeze dried instant coffee products are generally more expensive than spray dried/agglomerated instant coffee products.
In the United States, instant coffee products are typically consumed at about a 1% median coffee solid concentration. However, a significant portion of these consumers prefer a milder cup of coffee made at a lower concentration. Unfortunately, these consumers find it difficult to measure out current spray dried/agglomerated instant coffee products to achieve their desired flavor strength. The presence of fines in current spray dried/agglomerated products makes dosage control difficult and requires measuring out partial spoonfuls of product. This is both inconvenient and confusing to the consumer. The most practical way of delivering such a milder cup of coffee for these consumers is to reduce the bulk density such that a lower amount of coffee solids is dosed in each spoonful of product.
In addition, about half of the consumers of soluble coffee products prepare their drinkable beverage using a microwave. Typically, the water is first heated and then the desired dosage of product is added directly on top of the heated water. As noted above, freeze dried products do not dissolve very well when prepared in such fashion. This means the consumer must either accept a messy cup appearance or change their preparation habits.
To summarize, conventional coffee processing technology cannot readily meet the consumer need to deliver a lower and consistent spoon dosage that is readily soluble under all expected methods of consumer preparation. Although it is possible to make a low density spray dried/agglomerated soluble coffee product, such products have a tendency to break-up readily into undesirable fine particles. This makes spoon dosage control even more difficult for those consumers preferring a milder cup of coffee. Further, the manufacture of low density freeze dried products is difficult, expensive, and leads to poor solubility under certain preparation conditions, (e.g., when the coffee is added directly on top of hot water). There thus exists a significant consumer need to develop low density soluble coffee products that have less particle breakup, and exhibit better hot water solubility across a range of preparation practices.